Yellow pigments such as iron oxide hydroxide, cadmium sulfide, strontium chromate, lead chromate and some organic pigments, e.g., benzidine yellow, have been extensively used for many different applications. Most of the inorganic pigments usually of aforementioned cadmium sulfide, strontium chromate and lead chromate provide good thermal stability except iron oxide hydroxide. However, these inorganic, thermally stable pigments are relatively expensive, toxic and carcinogenic. Furthermore, environmental consequence is another concern.
In contrast to the aforementioned inorganic pigments, synthetic iron oxide, an FDA-approved pigment, is relatively non-toxic and environmentally friendly. Iron oxide hydroxide (α-FeOOH) is an inorganic yellow pigment, which exhibits general fastness properties against light, solvents, and hot weather. Furthermore, synthetic iron oxides are relatively low cost when compared to conventional pigments, such as those discussed above. Although yellow iron oxide is an economical yellow pigment, it lacks thermal stability.
Black iron oxide (Fe3O4) and yellow iron oxide (α-FeOOH) are dehydrated when heated above approximately 200° C. and then transformed into reddish brown hematite (α-Fe2O3) when the heating temperature is further increased to 230° C. For this reason, usage of yellow iron oxide is limited as a color pigment for the production of thermoplastics, such as high-density polyethylene, polypropylene, polycarbonates, etc., for which the processing temperature ranges from 200 to 300° C. Furthermore, the storage and transportation environments require special measures to prevent color change of black and yellow iron oxides due to dehydration occurring when stored at a relatively high temperature for a long period of time.
Since yellow iron oxide has the above-enumerated advantages when compared to conventional yellow pigments, it is desirable to perform further treatments of yellow iron oxide to enhance the thermal stability in order to constitute a useful alternative material.
Three main types of treatments have been proposed to overcome the problem of thermal instability of yellow iron oxide: (1) a hydrothermal treatment of yellow iron oxide in aqueous alkali solution to assist conversion of imperfect crystals to perfect crystal lattices (U.S. Pat. No. 3,969,494); (2) surface coating by precipitating other metal oxides/salts onto the surface of yellow iron oxide followed by a secondary hydrothermal treatment (U.S. Pat. Nos. 4,291,010, 4,374,676, 4,374,677 and 4,376,656); and (3) surface coating by precipitating other metal oxides/salts onto the surface of yellow iron oxide at ambient temperature (U.S. Pat. Nos. 4,053,325, 4,734,136 and 6,027,559). Aluminate is one of the most common compositions for the coating of yellow iron oxide to enhance the thermal stability.
In the cases of hydrothermal treatment, it is necessary to perform most of the treatments at relatively high temperatures and perform successive hydrothermal treatments. Therefore, the hydrothermal processes are relatively uneconomical. The table below summarizes some of the treatments for iron oxide pigments that have been developed in the past:
TABLE 1Summary of technology improve thermal stability of yellow iron oxideU.S.CoatingThermalPatentYearAssigneeCompositionStabilityProcess3,969,4941976Agency ofN.A.250°C.HydrothermalIndustrialtreatmentScience &Technology4,053,3251977PfizerFe(PO3)2252°C.Deposition of ironmetaphosphatecoating4,291,0101981Titan(FeAl)OOH283°C.Hydrothermaltreatment4,374,6761983Titanα-Sb2O3245°C.Deposition ofantimony oxidecoatingN.A.301°C.2nd hydrothermaltreatment(FeAl)OOH250°C.Hydrothermaltreatment4,374,6771983Titan(FeAl)OOH270°C.2nd hydrothermaltreatment(FeAl)OOH + Sb270°C.Hydrothermaltreatment4,376,6561983Titan(FeAl)OOH + Sb303°C.2nd hydrothermaltreatment4,734,1361988Bayer(AlO)xPO4(OH)x−3260-270°C.Deposition of(x = 3 − 10)aluminumphosphate coating6,027,5592000Toda(FeAl)OOH>265°C.Deposition ofaluminum ironoxide hydroxidecoating
There is a need in the art for cost-effective production of thermally stable yellow iron oxide. In particular, there is a need in the art for a method for depositing metal oxide/salt coatings on the surface of yellow iron oxide at ambient or low temperatures.
It is an object of the present invention to provide for an economical and practical method for coating oxide-based color pigments in order to enhance the heat-fastness and color performance of the color pigments.
Citation or identification of any reference in this section or any other section of this application shall not be construed as an admission that such reference is available as prior art for the present application.